Lead-acid batteries are a well known source of energy used in a variety of applications including, for example, automotive starting and industrial uses. The central structural elements of conventional lead-acid batteries are positive and negative grids coated with an active material to form plates, each plate having a lug and being separated from adjacent plates within a battery by porous separators. Typically, the battery grids are made from lead or a lead alloy and have a reticulated portion (reticulum) on which the active paste material is supported. The reticulum is bounded by a rigid border from which the lug(s) extend. As is known, the lugs are used for collecting grid current and joining plates of like polarity together in a cell. Thus, the grids serve as the framework and electrical contact between the positive and negative active materials which generally serve to conduct current. This conjoint electrochemical (corrosion) action and structural (load-bearing) role causes stress to the grids, particularly the positive grids. In most instances, failure of the battery occurs when the grids are no longer able to provide adequate structural support or current flow. Therefore, the primary properties of interest in the design and manufacture of battery grids are strength and resistance to both corrosive and mechanical stresses. Other properties to consider include the compatibility of the grid material with the active material (adherence) and various electrochemical and metallurgic effects.
Modernly, a large percentage of the battery grids used in commercially-available lead-acid batteries are manufactured by a process generically referred to as "continuous casting" (con-cast). Traditionally, continuous casting machines include a rotary drum having a reticulated grid pattern (i.e. mold cavity) machined into its outer peripheral surface, and a stationery shoe which overlays the grid pattern. The shoe functions both to dispense the molten lead into the patterned mold cavity and to scrape off any excess molten lead upon rotation of the drum. Due to rapid solidification of the molten lead, a continuous grid strip is removed from the drum upon rotation past the shoe. One example of a conventional continuous casting machine and the lead con-cast process associated therewith is disclosed in U.S. Pat. No. 4,349,067 issued to Wirtz et al.
Unfortunately, the continuous casting process suffers from several drawbacks which significantly limit its battery grid production capabilities. First, due to the rapid solidification characteristics of molten lead, large temperature gradients occur in the molten lead as it is discharged from the shoe and delivered across the entire width of the patterned mold cavity. Such temperature gradients result in significant variations in the grain structure of the lead upon its solidification. As is known, poor grain structure contributes to increased corrosion, decreased strength, disruption of the reticulum and eventual battery failure. To avoid such undesirable temperature gradients, it is typically required that the width of the shoe be relatively narrow which, in turn, prohibits or severely limits the capability of concurrently forming multiple grid strips. As such, the productivity (i.e. grids/min.) of continuous casting machines can not be increased above known processing limitations without compromising grid quality. Second, it is well known that conventional con-cast processing can not consistently form positive battery grids with the desired mechanical strength and resistance to mechanical and corrosive stresses. In particular, it is an industry belief that conventional con-cast processing is impractical for manufacturing positive battery grids because normal processing variations result in grids having an improper grain structure which, in turn, leads to increased corrosion and mechanical stress on the grids. Another consideration is the turbulence caused by the rotational speed of the casting mold. Increased turbulence contributes to mechanical defects in grid formation. Since positive grids differ from negative grids in that positive grids require greater strength due to anodic attack, they are generally formed with an increased cross-sectional thickness in comparison to negative grids. However, the increased mold depth required for casting positive grids is not conducive to con-cast processing because gradient cooling and flow turbulence caused during delivery of the molten lead typically results in poor grain formation. Moreover, the increased cross-sectional area required for positive grids requires the utilization of gratuitous lead. As such, lead battery grids are relatively expensive and contribute substantially to battery weight.
To alleviate these disadvantages, designs have been developed such as the one shown in U.S. Pat. No. 4,221,854, to Hammar et al., which discloses a lightweight laminated grid for use in lead-acid storage batteries. These laminated grids are formed by an expanded metal process which generally requires the perforation and expansion of a lead strip to form the open mesh reticulum. Unfortunately, grids formed by expanded metal processing generally are not high quality. Specifically, the quality is known to need improvement for use as positive grids. Thus, recognized deficiencies exist in the manufacture of battery grids which, at this point, have not been adequately addressed.